In article
,
Bonos Ego writes:
I once recall Philip Eden, correct me if I'm wrong, saying that a good
indicator of when Spring has arrived is when the average daily
temperature exceeds +6.0°c
snip

Yep. Presumably the soil temperature in the top inch or two will be
close to the average daily air temperature, and soil temperature is what
determines when many (most?) plants start growing.
--
John Hall
"Acting is merely the art of keeping a large group of people
from coughing."
Sir Ralph Richardson (1902-83)